Capacitors

It is a device which has an ability to store electric charge. It functions like a battery which charges and discharges efficiently.

A Capacitor consists of two metal plates separated by a dielectric/insulator. A capacitor may have more than two plates depending upon the they type of dielectric.

It is rated in Farads-Volts/meter, where Farads is unit of capacitor and volts/meter is the charge per distance. It is measured in capacitance.

The capacitor can store energy momentarily, but cannot add energy on continuous basis like battery. The capacitance[C] is the ratio of Charge[Q] to Voltage(V).

Capacitor removes ripples from DC signal, hence it is used in regulated DC power supplies.It allows high frequency current through it very easily. It provides less resistance to AC and high resistance to DC, hence we say that capacitor passes AC and Blocks DC.

Capacitor is one of the important component which you can visualise is various electronic device and circuits like in MOSFET's, Diode's, etc. where it plays an important role in their operation.

Important Concepts related to capacitor:

Quality Factor:

It is the ratio of energy stored to energy dissipated. Less is the dissipation of energy, higher is the Quality Factor of capacitor.

It is also defined as the ratio of reactance to the resistance of capacitor at a given frequency.

Capacitance Frequency Characteristics:

It is the graph which shows the relation of capacitance to its operating frequency.

Charging and Discharging:

In capacitor when it is provided with a supply, voltage starts building up in two ways: linearly or exponentially, depending upon its associated circuitry. This voltage builds upto maximum limit. Even after reaching max limit, if supply is still provided then it will damage the capacitor.

While charging if supply is removed at particular voltage and if there is no discharge path/open circuit then capacitor will hold that value of voltage(like energy stored in battery). When a complete path is provided then capacitor starts discharging and value of voltage across capacitor starts decreasing. Capacitor discharges in two ways like charging does.
This characteristic of capacitor is used in many applications like multi-vibrators, Oscillators, Digital Sampling etc.

Equivalent Series Resistance(ESR):

It is the characteristic representing the sum of resistive (ohmic) losses within a capacitor.It is the AC impedance of the capacitor when used at high frequencies and includes the resistance of the dielectric material, the DC resistance of the terminal leads, the DC resistance of the connections to the dielectric and the capacitor plate resistance all measured at a particular frequency and temperature. While ESR is undesirable, all capacitors exhibit ESR to some extent. ESR is a frequency dependent characteristic. By considering Ohm's law: Vr = IR, where Vr is the ripple voltage and R is the ESR. It is noticed that if the current (I) increases from 2A to 10A then the ripple voltage will also increase. Thus ESR should be as low as possible.

Classification of capacitors

The Capacitor are classified in three ways:

1.Based on nature of value

2. based on dielectric material

3. based on polarization

Electrolytic Capacitor

Electrolytic Capacitor is a polarised capacitor that uses electrolyte to achieve larger value of capacitance.

This capacitor is formed by placing the materials in the following sequence/stack: First is aluminium foil, followed by electrolyte forming a cathode after which there is a thin layer of oxide followed by metal aluminium foil forming an anode of the capacitor.

This arrangement is then folded along with a paper foil as insulator into a cylindrical case taking out its connecting pins.

Electrolytic capacitors does not perform well with large amplitude and high frequency signals. This is because of power dissipation at the parasitic internal resistance known as Equivalent Series Resistance (ESR).

They are available in range of one micro Farads up to several 1000's micro Farads.

These capacitor have marked polarities on its cap. One can also identify its polarities through the length of their pins/terminals. A Pin which is large in length is positive terminal and the one which is small is a negative terminal.

Applications:

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